1. Field of the Invention
The present invention relates to the field of injection molding. More particularly, the invention relates to insulating nozzle tips used, for example, in thermoplastic molding.
2. Description of the Related Art
Injection molding takes plastic pellets and converts them to a broad array of useful items such as bottles, other types containers, and toys. One example of a device for performing this process has a hopper for holding a quantity of the pellets. A passage links the hopper with a manifold. In the passage, the pellets melt through the action of a screw pushing the pellets along the passage, and heaters warming the passage. The molten plastic reaches the manifold, and is injected through one or more nozzle assemblies into a mold. Each nozzle assembly has an opening for receiving the molten material from the manifold. An elongated portion of the nozzle assembly guides the molten material to its tip. Each nozzle tip has one or more orifices for ejecting the molten material into the mold. A heating assembly in each nozzle assembly maintains the plastic material in a molten state until it is injected into the mold.
The entry orifice in the mold for receiving the molten material, known as a xe2x80x9cgate,xe2x80x9d is sized to accommodate at least a part of the nozzle assembly. Once the molten material reaches the mold, it is rapidly cooled to form the desired shape. To facilitate cooling of the molten material, some molds incorporate channels through which cooling water flows.
In practical application, a number of molds may be arranged in a turret, with the nozzle assembly positioned so that it can be inserted at the gate to the mold. The molten material is then injected into the mold through one or more orifices in the distal end of the nozzle assembly, the nozzle assembly is removed from the mold, and the turret turns a predetermined distance to align the gate of another mold with the nozzle assembly. This process then repeats.
Where the distal end (the tip) of the nozzle assembly nears the mold, the cooler mold may reduce the temperature of the nozzle tip, and hence the temperature of molten material. This causes two problems:
(1) the molten material cools and hardens somewhat, clogging the nozzle orifice(s);
(2) a xe2x80x9cgate bubblexe2x80x9d develops, formed on molten material leaked because the seal between the nozzle tip and the mold fails.
To avoid these problems, the nozzle tip is frequently insulated. For example, U.S. Pat. No. 5,569,475 to Adas et al. describes a thermal insulator between the nozzle assembly and the surrounding molding plates. This patent describes using, preferably, a ceramic insulator such as zirconia oxide. A thin layer of the zirconia oxide is sprayed onto either the opening in the mold plate, or onto the nozzle body. If a spray-on coating is used, the nozzle body can first be roughened, and an undercoat, such as nickel-aluminum, applied to the nozzle body to assist with bonding. In addition, a protective coating may be sprayed onto the insulator layer. The protective coating, according to the patent, should be a wear-resistant and machinable material, preferably a metal such as titanium, nickel, or molybdenum. This metallic protective layer then contacts the mold.
Other sources also teach insulating a nozzle. For example, U.S. Pat. No. 5,474,439 to McGrevy discloses a cap (a titanium insulator) tightly disposed on the nozzle body by heating. The cap has a gate well, an opening that aligns with the orifice of the nozzle. The cap has projections with indentations therebetween on an outer surface of the cap. The projections abut the mold. The indentations are filled with air, and these air gaps help to maintain the projections abutting the mold at an ambient temperature even when heated fluid flows through the gate wells. This, the patent asserts, maintains the desired relationship between the projections and the mold.
U.S. Pat. No. 5,324,191 to Schmidt discloses a sealed edge gate for an injection molding system. In response to the gate bubble problem above, the Schmidt patent describes placing a seal ring around the tip end of the nozzle housed within a recess in a mold plate. The seal ring, according to a preferred embodiment, is made of a material with a lower thermal coefficient of expansion than that of the nozzle material. For example, the seal ring could be made of titanium, where the nozzle is made of steel or copper alloy. With this arrangement, the expansion of the nozzle will press the seal ring outward against the wall of the mold plate, and will more tightly grip the seal ring. Bubble grooves may be included in the seal ring, the grooves communicating with the nozzle orifices. In this way, plastic material fills the groove and acts as a thermal insulator to minimize the heat transferred via the seal ring to the mold cavity plate.
Another approach, described in U.S. Pat. No. 4,662,837 to Anderson, provides a thermally insulative sleeve for an injection molding apparatus. The sleeve has two components. The first component is elastically yieldable. The second component, located upstream of the first component, is rigid. That is, the two components of the sleeve are arranged serially along a longitudinal direction of the nozzle. The first component may be made of an elastomeric, fluoroplastic, or silicone material which does not degrade at the operating temperature of the nozzle. The second component may be made of liquid crystal aromatic polyester copolymers, polyimides, polyethersulfones. The second component may be termed a back-up ring and provides alignment between the nozzle and the die cavity, while being sufficiently rigid to keep the first component from being displaced.
However, challenges remain in sealing and insulating the interface between the nozzle and the mold. For instance, Vespel(trademark) is useful as an insulator since it is rated to a continuous use temperature of approximately 260xc2x0 C. [500xc2x0]. However, many resins are now processed at higher temperatures which cause polymer degradation and cracking of the known Vespel(trademark) nozzle tip insulator.
Titanium is not an optimum insulator since it conducts too much heat and because it does not always seal properly (e.g., titanium can withstand only a small preload, such as 0.1 mm). If the hot tip positions and/or the gate detail ball dimensions are out of specification, the preload that a titanium insulator can withstand becomes even smaller, and a gap may be created.
With such an imperfect seal, a xe2x80x9cgate bubblexe2x80x9d may form in which molten material leaks from the orifice(s) of the nozzle. This leaked material then cools and becomes less fluid. Consequently, the material may clog the nozzle orifice(s) and/or seal the gate opening in the mold, interfering with the molding process.
It is an object of the invention to create a nozzle tip insulator that overcomes the drawbacks of conventional nozzle tip insulators.
To this end, one aspect of the invention comprises a nozzle tip insulator having inner and outer annular portions. The inner annular portion contacts the injection molding nozzle tip, and the outer annular portion contacts the inner annular portion. The outer annular portion is less thermally conductive than the inner annular portion. That is, the insulator has a stepped heat gradient profile from an inside to an outside thereof.
In another aspect, the invention is directed to an injection molding nozzle tip insulator including an inner conductive portion and an outer insulative portion. The outer insulative portion surrounds the inner portion, and an inner surface of the outer portion contacts an outer surface of the inner portion along at least a portion of the surfaces. The inner conductive portion may be, or include, titanium. The outer insulative portion may be, or include, Vespel(trademark).
According to still another aspect, the present invention is directed to a nozzle tip insulator for use with an injection molding nozzle tip. The insulator according to this embodiment includes an inner conductive ring and an outer insulative ring. The outer insulative ring is concentric with the inner ring, with an inner surface of the outer ring contacting an outer surface of the inner ring. The outer insulative ring is less heat-conductive than the inner conductive ring.
Yet another aspect of the invention is an injection molding device including a mold with an opening, and a nozzle assembly for contacting the opening. The nozzle assembly includes a nozzle tip having at least one opening through which molten material passes, and a nozzle tip insulator surrounding the nozzle tip. The nozzle tip insulator, in turn, includes an inner conductive portion, and an outer insulative portion surrounding the inner portion. An inner surface of the outer portion contacting an outer surface of the inner portion along at least a portion of the surfaces.
A further aspect of the present invention encompasses an injection molding nozzle tip insulator including inner means, in contact with a nozzle tip, for conducting heat, and outer means for insulating the heat conducted by the inner means. The outer means surrounding the inner means, an inner surface of the outer means contacting an outer surface of the inner means along at least a portion of the surfaces.
Another aspect of the present invention is directed to a method for insulating an injection molding nozzle tip. The method includes steps of (1) providing an inner annular portion in contact with an injection molding nozzle tip, and (2) providing an outer annular portion in contact with the inner annular portion, the outer annular portion being less thermally conductive than the inner annular portion so that the insulator has a stepped heat gradient profile from an inside to an outside thereof.
An additional aspect of the present invention encompasses a method for insulating an injection molding nozzle tip, including the steps of (1) providing an inner conductive portion of a nozzle tip insulator, and (2) providing an outer insulative portion of a nozzle tip insulator surrounding the inner portion, an inner surface of the outer portion contacting an outer surface of the inner portion along at least a portion of the surfaces.
The invention is described in more detail below with reference to the accompanying drawings.